<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        let num1 = +prompt('请输入银行卡余额')
        let num2 = +prompt('当月食宿消费金额')
        let num3 = +prompt('当月生活消费金额')
        function fn(a, b, c) {
            return a - b - c
        }
        console.log(fn(num1, num2, num3));
    </script>
</body>

</html>